Enhancement of Feammox nitrogen removal performance and mechanism by activated carbon as an electron shuttle

Abstract

Introducing exogenous electron shuttles (ESs) that could synergistically enhance the removal efficiency of both ammonium nitrogen (NH₄⁺-N) and total nitrogen (TN) in Feammox systems. In this study, stable operation of a Feammox reactor was accomplished through acclimation of Feammox sludge. The removal rates of NH₄⁺-N and TN reached approximately 75% and 50%, respectively. The coupled Feammox and nitrate-dependent ferrous iron oxidation (NDFO) processes constituted the primary nitrogen removal pathways. Compared to the blank control group and the commonly used anthraquinone-2,6-disulfonic acid (AQDS), the addition of activated carbon (AC) significantly enhanced NH₄⁺-N and TN removal efficiencies by 30.61% and 18.21%, and 9.01% and 10.34%, respectively. Mechanistic studies indicated the formation of FeOOH in the presence of FeCl₃. The abundant quinone (C=O) and phenolic hydroxyl (–OH) functional groups on the AC surface effectively promoted the recycling of Fe(II)/Fe(III), thereby enhancing electron transfer efficiency in the Feammox system. Additionally, AC stimulated the secretion of microbial extracellular polymeric substances (EPS), further accelerating electron transfer between Fe and NH₄⁺-N. Notably, compared to soluble ESs like AQDS that are prone to loss via water flow, AC as a solid-phase ES exhibits superior stability and persistence, demonstrating enhanced nitrogen removal performance. This study developed a green, cost effective and efficient biological nitrogen removal technology, providing significant theoretical support and technical reserves for the practical engineering application of Feammox.

Graphical abstract

Keywords

Activated carbon / Biological nitrogen removal / Electron shuttle / Feammox / Nitrogen-containing wastewater

Highlight

	● ESs enhance the removal efficiency of both NH4+-N and TN in Feammox systems.
	● AC has higher denitrification removal efficiencies than AQDS.
	● The abundant C=O and –OH promoted the recycling of Fe(II)/Fe(III).

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